Coriolis type mass flow meters operate on the principle that fluent material flowing through a conduit, when exposed to a deflection transverse to the flow or motion of the material, will react with a measurable force (the Coriolis force) on the conduit walls. The Coriolis reaction is generated by the flow moving in an instanteously changing curvilinear path and acts with a force which is directly proportional to the mass flow rate through the conduit. Because the Coriolis force is dependent only on the mass rate of flow and the oscillation rate and its effect on the tubing is an integration of the force generated along the length of the tubing, the Coriolis reaction measurement is independent of the physical properties of the fluid, such as density and velocity. By segregating the effect of the Coriolis reaction from the overall conduit motion during operation, a determination of the corresponding mass flow rate may be made.
Known Coriolis type mass flow meters are susceptible to unwanted external noise which influences the conduit motion and the sensing of the Coriolis reaction. Additionally, known meters often compromise structural properties to increase sensitivity of the meter to the Coriolis reaction.
One version of commercially available Coriolis type mass flow meter is produced by the Micro-Motion Corporation and is exemplified by the teachings of U.S. Pat. No. 4,422,338, 4,491,025 and Re. 31,450. Each of these patents show a substantially U-shaped conduit structure having a rigidly mounted flow tube which projects from the fixed mounting position so as to be cantilevered from the mount. The bight end of the relatively stiff cantilevered structure is oscillated preferably perpendicular to the plane of the U-shape and the Coriolis reaction is measured on each of the opposing legs of the U-shape.
The U.S. Pat. No. '025 patent, referred to above, as well as U.S. Pat. No. 4,127,028 show a conduit structure having two identically shaped, adjacently positioned, cantilevered U-shaped flow tubes which create a tuning fork effect during oscillation. Each of the U-shaped flow tubes receives an equivalent and parallel flow and is oscillated in an opposite mode. The deflection of the flow tube due to the Coriolis reaction force on the two oppositely oscillating U-shapes, is also in an opposing direction with the Coriolis reaction at all adjacent points being substantially equal in magnitude. Sensors mounted between the adjacent legs of the two U-shaped tubes measure substantially twice the deflection due to the Coriolis reaction at any one position on the tubes due to this opposing relationship.
Another commercially available version of a Coriolis type mass flow meter is produced by the Exac Corporation and includes a conduit having two projecting tubular loops having long attachment arms which are connected to linearly positioned fixed locations. This type conduit design is comparable to that shown in FIG. 5 of U.S. Pat. No. 4,127,028 referred to above. The tubular loops project away from the line formed between mounting positions of the inlet and the outlet and provide an appearance of being cantilevered but are not rigidly mounted at the base of the projection. This structure creates a flexible flow tube structure by incorporating a torsional bending response of the flow tube along with the deflections due to the Coriolis reaction force. This flexibility of the flow tube structure increases the measureability of the Coriolis reaction force on the conduit.
Existing designs often compromise conduit wall thickness or use long inlet and outlet flow tube sections in order to increase measurement resolution of the conduit deflections due to the Coriolis reaction and to increase the overall sensitivity of the meter. Both of these approaches lead to disadvantages in the final flow meter product. In one case, the flow meter will be limited due to structural weaknesses and, in the other, the increase in Coriolis sensitivity will be coupled with an increased susceptibility to noise, as well as detrimental effects due to added weight of the flow tubing.
Cantilevering a flow tube away from a fixed mounting point (Micro-Motion) or projecting a flexible flow tube loop away from its inlet and outlet (Exac) suspends the flow tube (and the mass of the fluid within the tube) away from the conduit mount. The center of gravity of the flow is also projected or suspended away from this mounting position, making the conduit structure unstable with respect to vibrations from the environment of the flow meter which are transmitted to the conduit shape along with the applied oscillation of operation. In some instances the instability of the suspended weight of the conduit will increase the effect of unwanted vibrations on the flow tube as well as contaminate the Coriolis reaction measurement. External noise, which is produced by mechanical movement along the pipe line or in the local environment of the flow meter, often has an overall vibrational effect on the meter at a frequency close to the fundamental vibrational frequency of the conduit. Additionally, inaccuracies in the oscillatory motion of the flow tube due to inconsistencies in the conduit structure, as well as the nature of the applied oscillatory motion itself, further contaminate the Coriolis measurement. Additional vibrations over those applied during operation of the meter and motion of the flow tube which is not a result of the Coriolis reaction to the applied vibrations manifest themselves as unwanted noise in the signals produced by the flow meter sensors. These unwanted vibrations as well as the suspension of the center of gravity greatly affect the mass flow determination and the accuracy of the meter.
An additional problem found in known Coriolis mass flow meters relates to increase in sensitivity by providing long flow tube extension sections adjacent the inlet and outlet to the flow meter to increase flexibility of the flow tube and to increase the measurability of the Coriolis reaction force. These extensions can be seen in both U.S. Pat. No. 4,127,028 and U.S. Pat. No. 4,559,833, showing S-shaped flow tubes. Extension sections add to the overall weight of the meter and suspend the weight of the flow tube from its mounting. Also, restrictions may be introduced within the conduit flow path via these extensions which may further limit the usefulness of the meter in certain applications as well as limit the overall accuracy of the meter. In turn the natural frequency of the applied oscillation is typically lowered to achieve a more easily determined displacement of the conduit due to the Coriolis reactions.
Internal flow restrictions are also found in the commercially available flow meter produced by the Smith Meter Company, the assignee of U.S. Pat. No. 4,559,833, which includes an "S" shaped conduit positioned transverse to the line of the inlet and outlet. This conduit structure includes an elbow portion which directs the flow into and out of the "S" shape, which is positioned transverse to the inlet and outlet, and includes relatively tight turns within the "S" tube portion. The flow restrictions created by this structure may be detrimental to the flow, moving through the conduit such that certain fluids may not be applicable for measurement by Coriolis type meters. Additionally, the weight of the horizontally positioned "S" shaped conduit is suspended between its supports such that a component of the weight of the fluid and the tubing resists the applied oscillatory motion of the meter as well as the Coriolis reaction. The suspended weight of the flow tube also creates an unstable structure which is subject to external noise influences. Commonly assigned and co-pending application Ser. No. 809,658 filed Dec. 16, 1985, now U.S. Pat. No. 4,716,771 issued Jan. 5, 1988, which is herein incorporated by reference, teaches a flexible conduit of relatively long length which is spiralled about the axis defined by its inlet and outlet. The loop of the continuous tubing is, ideally, positioned substantially transverse to the flow of the defined fluid stream. The intent of this flexible design is to remove the effect of transverse oscillations from the conduit environment while providing an extremely flexible structure which will enhance Coriolis reaction sensitivity.
A typical problem in evaluating the function and resultant motion of the flow meter geometry relates to the nomenclature utilized to describe the operation and function of the conduit in response to both the oscillation of the conduit transverse to the flow as well as the deflections due to the Coriolis reaction of the fluid on the conduit. The language utilized to describe the function of the flow meter is typically related to a fluid particle which moves through the flow tube and is then continued when describing the motion of the rigid body of the flow tube. In describing the U-shaped conduits, as found in the reissue patent, the U.S. Pat. No. '028 patent, the U.S. Pat. No. '025 patent and the U.S. Pat. No. '338 patent (referred to above), references are also made to "a fixed axis" about which the U-shaped conduit is "rotated". The original Coriolis mass flow meters of the 1950's (as illustrated by Pearson U.S. Pat. No. 2,624,198, Roth U.S. Pat. No. 2,865,201, etc.) utilize rotating conduits or rigid circular shaped flow tubes which are vibrated to produce the Coriolis reaction force in response to the rotation of the flow through the flow tube. There is a tendency, because of the developmental history of Coriolis mass flow meters, to describe vibrating or oscillating type Coriolis flow meter conduits as also rotating about a deflection and/or oscillation axis. However, the bight end of a cantilevered beam does not "rotate" about its fixed mounting point but, rather, deflects at all points along the conduit length in a non-circular and non-uniform manner about this mounting position. Additionally, since the flow meter conduit is, typically, positioned within a pipeline or a defined fluid stream, which is not rigidly fixed, the fixed mounting points will also move in response to both the applied oscillation of the flow meter as well as in response to external vibrations and noise created in its environment. This essentially non-rotational relationship is also found in a projecting loop type conduit structure since the extension portions of the loop will flex in a non-uniform or non-circular manner and since the projection is not rigidly mounted to a fixed structure.
The oscillation of the bight end of a cantilevered U-shaped conduit, as well as a projecting loop type structure (which is not essentially cantilevered in that it is not fixed to a rigid structure), are not properly referenced to circular type rotation. Additionally, the deflection or conduit movement due to the integrated Coriolis reaction forces on opposite sides of the applied oscillation are also not purely rotational at all positions on the conduit about a single (deflection) axis. Therefore, flow tube and sensing structures as known in the art which relate to the rotational aspects of the meter and passage of the conduit through stationary planes often do not produce the advantages in Coriolis force measurement as contemplated by their theoretical calculations.
Additional factors affecting the sensitivity and accuracy of known Coriolis type mass flow meters relate to the frequency of oscillation as applied to the flow meter conduit to create reaction. The flow meter conduits are essentially rigid structures which are oscillated at their fundamental natural vibrational frequency. However, machinery in the external environment of the Coriolis mass flow meter may create an external influence on the conduit structure which may significantly affect the readings made by the sensors used to measure the Coriolis reaction and, therefore, significantly reduce the accuracy and reliability of the mass flow determination. Additionally, since all known Coriolis type flow meters are operated at their fundamental resonant frequency or the first harmonic, contamination from these outside influences is always a factor on the sensitivity of the meter.
The geometry and the vibrational characateristics of known Coriolis flow meters often limit the measurability of the Coriolis reaction force and/or the overall sensitivity of the flow meter. More flexible flow tubes are desirable to increase sensitivity but flexibility often is provided at the expense of accuracy or overall performance characteristics. By reducing the thickness of the conduit wall of the flow tube to create a more flexible structure, the longevity and the operational safety of the meter may be compromised.